It's based on data from the U.S. Department of Transportation for
on-time performance, denied boardings, mishandled baggage, and
customer complaints. Each category is weighted to reflect its
importance to passengers.

As a whole, the U.S. airline industry posted its best score since
1991, despite losing ground in on-time performance and baggage
handling. Only airlines that handled at least 1% of domestic
scheduled-service passenger revenues were included.

Here's the final ranking for 2013, with each airline's 2012 rank
in parentheses. Most carriers didn't move too far in the past
year, though AirTran dropped from third to tenth: